Objectives: Vestibulo-cochlear implants are a potential treatment approach for patients with loss of vestibular function. A critical aspect is the development of reliable and objective outcome measures, to assess their efficacy and optimize their performance. As in cochlear implant care and other neural prostheses, electrically evoked compound action potentials (eCAP) can be used to measure the neural response to implant stimulation. eCAPs might provide information about the presence and misalignment of the electrically evoked vestibulo-ocular reflex, and potential vestibulo-cochlear interactions. This could be valuable in intra-operative functionality assessment, as input for fitting, and to measure spread of excitation. This study aimed to explore the potential role of eCAPs in vestibulo-cochlear implant research.

Design: eCAPs were measured in ten subjects with a vestibulo-cochlear implant. Different configurations were used, i.e., trans-canal, vestibulo-cochlear and cochleo-vestibular. The predictive value of eCAPs was evaluated for four outcomes: presence of an electrically evoked vestibulo-ocular reflex (eVOR), degree of misalignment of the eVOR, auditory perception due to vestibular stimulation, and the prevalence of vestibular activation due to cochlear stimulation.

Results: The results demonstrated a high positive predictive value of the eCAP for the presence of an eVOR (i.e., 1.0), while the negative predictive value was low (i.e., 0.54). The presence of trans-canal eCAP did not correspond with the degree of misalignment in the eVOR. Furthermore, the predictive values for auditory perception were low (i.e., ≤0.5). eCAP recordings in the cochlea to vestibular configuration imply a high likelihood of cochlear to vestibular interaction (i.e., 67% of electrodes).

Conclusions: The presence of a vestibular eCAP was demonstrated to be a predictor of the presence of the eVOR. However, the low negative predictive value prevents the eCAP from providing a reliable indicator for intra-operative electrode position evaluation. eCAP measurements in vestibulo-cochlear configurations indicated that spread of excitation between the cochlea and canals is already possible at stimulation levels within the clinical fitting range of both the vestibular and cochlear electrodes. Measured eCAPs did not correspond with misalignment of the eVOR or the presence of an auditory percept as result of vestibular stimulation. Future research is needed to elucidate the full potential of vestibulo-cochlear implant eCAP measurements.